The present invention generally relates to phase-locked oscillators, particularly to a phase-locked oscillator employing a phase-locked loop, and more particularly to a phase-locked oscillator employing a phase-locked loop, which is suitable for applications requiring conservation of electric power such as in a portable radio receiver/transmitter, and the like, and in which the phase-locked loop is intermittently operated.
Among means for saving average electric power consumption on radio receiver/transmitter, especially on portable radio receiver/transmitter, means of intermittently operating part of circuits of phase-locked oscillators requiring relatively large electric power have been described in Japanese Patent Unexamined Publication Nos. 58-66434 and 58-159029. Generally such a phase-locked oscillator comprises a reference oscillator, a reference frequency divider for dividing the frequency of output clocks of the reference oscillator, a voltage-controlled oscillator (hereinafter abbreviated to "VCO"), a frequency divider for dividing the frequency of the output clocks of the VCO, a phase comparator for comparison in phase between the frequency-division carry-signals of the frequency dividers, and a loop filter for smoothing the output error signal of the phase comparator and for feeding a VCO control voltage. In the above-mentioned prior art references, the average electric power consumption saving is attained by wholely or partially interrupting the electric power supply for the circuits except the VCO. In this case, a voltage-hold circuit for holding a control voltage to be applied to the VCO (that is, the output of the loop filter) to a value just before the cut-off of the electric power supply is provided to keep the output frequency of the VCO constant during the cutting-off of the electric power supply.
However, no consideration has been given to the fact that the two output signals of the reference frequency divider and the frequency divider to be fed to the phase comparator should be made to be in phase whenever the electric power supply is applied again from its off-state.
Each of the reference frequency divider and the frequency divider generally includes a counter constituted by one or more bistable trigger circuits or flipflops. The number of internal states in a binary counter constituted by a plurality of flip-flops, n in number, is 2.sub.n, and the internal state of the counter at the time of turning on of the electric power supply is indefinite. In other words, the number of input clock signals in a period from the turning on of the electric power supply to the output of the first carry is within a range of 2.sub.n but indefinite whenever the electric power supply is turned on. The electric power supply is cut off by an intermittent operation from the phase-locked state. For this reason, the probability that the respective signals fed to the phase comparator at the time of turning on of the electric power supply will become in phase is very little even if the output frequency of the VCO does not change during the off-state of the electric power supply, so that, in general, a phase difference occurs between the signals. Because the phase difference after turning on of the electric power supply produces a large error signal to be applied to the VCO through the loop filter, the frequency of the VCO is temporarily widely changed. Thereafter the frequency of the VCO is returned to a locked state by a general pull-in effect. The use of such an intermittently operative phase-locked oscillator as an oscillator for a receiver of a radio receiver/transmitter causes a problem in that unreceivable conditions temporarily occur because of the shifting of the frequency of the VCO every time the electric power supply is turned on in the intermittent operation.